Computational study of hydrogen-bonded complexes of HOCO with acids: HOCO?HCOOH, HOCO?H _2SO _4, and HOCO?H _2CO _3

摘要

Quantum chemistry calculations at the density functional theory (DFT) (B3LYP), MP2, QCISD, QCISD(T), and CCSD(T) levels in conjunction with 6-311G(2d,2p) and 6-311G(2df,2p) basis sets have been performed to explore the binding energies of open-shell hydrogen bonded complexes formed between the HOCO radical (both cis-HOCO and trans-HOCO) and trans-HCOOH (formic acid), H _2SO _4 (sulfuric acid), and cis-cis-H _2CO _3 (carbonic acid). Calculations at the CCSD(T)6-311G(2df,2p) level predict that these open-shell complexes have relatively large binding energies ranging between 9.4 to 13.5 kcalmol and that cis-HOCO (cH) binds more strongly compared to trans-HOCO in these complexes. The zero-point-energy-corrected binding strengths of the cH?Acid complexes are comparable to that of the formic acid homodimer complex (～13-14 kcalmol). Infrared fundamental frequencies and intensities of the complexes are computed within the harmonic approximation. Infrared spectroscopy is suggested as a potential useful tool for detection of these HOCOAcid complexes in the laboratory as well as in various planetary atmospheres since complex formation is found to induce large frequency shifts and intensity enhancement of the H-bonded OH stretching fundamental relative to that of the corresponding parent monomers. Finally, the ability of an acid molecule such as formic acid to catalyze the inter-conversion between the cis- and trans-HOCO isomers in the gas phase is also discussed.